The invention is concerned with a gas turbine engine combustion system and with means for mixing fuel and air in a gas-fuelled engine, particularly gas turbine engines using gas fuel of low calorific value.
Fuel-air mixing means (burners) to provide the combustible medium for gas turbine engine operation take many and varied forms according to manufacturer preference. A manufacturer may become expert in a particular burner type and wherever possible will adapt that type of burner to suit the engine duty, for example to burn unusual or particular kinds of fuel.
The present applicant has already devised a combustion system which incorporates a burner of the radial inflow swirler type. It is sometimes desirable to able to burn a fuel gas of low calorific value (LCV fuel), from say a coal gasification process. Difficulties in the use of such fuel include the volume of fuel required for a given power output being comparatively large in relation to the volume of air when compared to, for example, high calorific value (HCV) liquid fuels. Between these extremes, there are significant differences in respect of, among other things, fuel injection position, direction of flow and flow rates in order to achieve best mixing of air and fuel. Also, where an LCV fuel has a relatively high flame speed, flame speed being the rate at which a flame will propagate in a mixture (which is fast for example where it contains a high proportion of hydrogen), there is higher risk of fuel pre-ignition. When this occurs in parts of the burner not intended to accept a flame, damage may be caused to components of the burner.
Burner designs which encourage small regions of re-circulating air/fuel mixture to form in proximity to a burner component surface may be harmful because a flame may become stabilized in such a region, being effectively static. It may then attach itself to the burner surface and burn it away.
It will be understood by the skilled person that LCV fuel being of low calorific value may comprise in the region of 20-60% of the air-fuel volume in order to achieve required engine power. Plainly, introducing large amounts of fuel into an inflow swirler system presents quite different problems to that of HCV fuels where lower volumes are more usually applied to such systems.
There are two main options open to the skilled person to achieve the correct volume of fuel for mixing with air. Either the fuel must be injected through small openings at relatively high pressure into the air-stream or it may be injected through large openings at relatively low pressure. While high pressure flow through small openings may be typical for HCV fuels, low pressure flow through large openings is untypical.
It has been found that injecting large amounts of fuel through small openings at higher pressure induces turbulence in the air/gas stream and this is especially so where the fuel is injected at some angle to the air-stream. Whereas this may be advantageous when dealing with low volume, high calorific value HCV fuels (where it may promote better mixing), it is found detrimental for LCV fuels and especially so where such fuels have relatively high flame speed. As already mentioned, in such cases a flame may become established in a re-circulation region (effectively a static region) and then attach itself to an edge of the swirler hardware, for example at the trailing edges of vanes. Should this happen, the flame may eventually burn away the metal.
In addition to difficulties associated with specific fuels, all new gas turbine combustion systems must meet ever more restrictive environmental pollution standards in relation to combustion exhaust products discharged to atmosphere.
It is therefore an aim of the preferred implementation of the present invention to provide a burner of the radial swirler inflow type which satisfactorily mixes LCV type gas fuels with air to enable controlled combustion in a downstream combustion chamber and which results in engine exhaust pollution levels, in particular CO, within acceptable limits.
Accordingly, in order to overcome the problems associated with known burners, the present invention provides, in one aspect, a gas turbine engine combustion system, comprising in flow sequence a radial inflow swirler for mixing gaseous fuel and air, a combustion pre-chamber and a combustion main chamber, the swirler, the pre-chamber and the main chamber having a common longitudinal axis, the swirler comprising air and gas fuel passages angularly arrayed around the pre-chamber, the passages being oriented tangentially to a notional circle centered on the common longitudinal axis, thereby in operation to impart a common swirling motion to streams of fuel and air as they enter the pre-chamber from the passages, each gas outlet passage having an exit situated immediately downstream of an exit of an air supply passage with respect to the direction of swirl and being sized relative to the air supply passage such that at least at a predetermined power condition of the engine, the mass mean velocity of the gas- and air-streams at said notional circle are closely matched to each other.
Each gas fuel passage preferably includes means for restricting the fuel flow. The restricting means may comprise a narrow, i.e., reduced cross-section, portion of the fuel passageway, preferably at the entrance to the fuel passageway.
The ratio of the area of the restricted or narrow portion of the fuel passageway to the remainder of the passageway may be in the range from 1:1.1 to 1:1.7 and is preferably 1:1.4.
The passageways are preferably at an inclined angle to radii of the swirler so that the passageways emerge at the radially inner ends tangentially to a notional circle centered on the same axis as a combustion pre-chamber located downstream of the mixing means. The diameter of the notional circle is preferably between 0.7 and 1.0 times the diameter of the combustion pre-chamber.
In a second aspect of the invention, there is provided fuel/air mixing means for incorporation in the burner of a gas-fuelled engine, the mixing means comprising fuel passageways and air passageways for introducing fuel and air to a combustion chamber from a radially outer position to a radially inner position relative to an axis concentric with the combustion chamber, each gas fuel passageway having an exit situated immediately downstream of an exit of an air passageway with respect to a direction of swirl of the fuel and air in the combustion chamber, the radially inner ends of said passageways being substantially tangential to a notional circle centred on the same axis as said chamber. Again, the gas fuel passageways are preferably sized relative to the air passageways such that at least at a predetermined power condition of the engine, the mass mean velocity of the fuel and air at said notional circle are closely matched to each other.
The fuel and air passageways preferably alternate circumferentially around said axis. The passageways are also preferably disposed at inclined angles to radii of a radial swirler inflow type mixing means.
In one embodiment of the invention, each fuel gas passageway includes means for smoothing the flow of the gas. The smoothing means also acts as a restrictor and may comprise a plate extending across the passageway and having a plurality of apertures therethrough. The apertures are suitably circular, although other shapes may alternatively be employed, and they may be arranged in a grid pattern or randomly. Twelve apertures are suitably provided in each plate, although more or fewer apertures may be used. The plates are conveniently located in opposed grooves in the side walls of each passageway at a position intermediate the ends thereof. While it may be desirable to secure the plates in position permanently, for example by welding, it may alternatively be convenient for the plates to be mounted in the grooves removable, to permit their replacement with plates of an alternative configuration in the event of a change of fuel gas, for example.
The invention also comprehends a gas-fuelled gas turbine engine comprising fuel/air mixing means as set out in any of the preceding paragraphs.